(1) Field of the Invention
This invention relates to a cell switch module used in an ATM switching board of the broadband ISDN, especially to a growable interconnect fabric cell switch module comprising cell switches connected to one another in a multiple-step architecture and also to a method for arbitrating cell transfer paths by use of the growable interconnect fabric cell switch module.
(2) Description of the Prior Art
FIG. 1 shows a construction of an ATM switching board used in an ATM switching channel system. Each element of the ATM switching board functions as follows.
Each line interface (LIF) 2301, which is provided for each line connected to the ATM switching board, carries out O/E conversion, E/O conversion, S/P conversion, P/S conversion, cell synchronization, header conversion, traffic monitoring and traffic control.
ATM switches (SW) 2302, for exchanging ATM cells, each has a 32.times.32 construction, are interconnected in a three-step architecture to form a 1024.times.1024 switch module. In theory, cells are transferred with no conflict in whichever way the 1,024 input lines and the 1,024 output lines are combined if an optimum path is provided.
Each line interface 2301 inputs and outputs an STM-1 signal having a transfer speed of 155.52 M bit/sec. The ATM switching board receives 1,024 STM-1 signals of this speed in total. The above speed allows a transfer of a fast and large-capacity digital signal such as a high-speed data and a TV signal.
In recent years, there has been a demand for a transfer capacity several times larger than 155.52 M bits/sec. in order to deal with, for example, a high definition television signal. Such a signal should be transferred as an STM-4 signal, which is obtained by concatenation-multiplexing four STM-1 signals. The STM-4 signal, however, cannot be interfaced by a switching board only with a capability of dealing with STM-1 signals.
In order to overcome such an inconvenience, a VC4-4C signal of an STM-4 payload as shown in FIG. 2a is divided into four VC-4 signals of an STM-1 payload as shown in FIG. 2b by a multiplexing device (not shown) and transferred to an identical destination before being concatenation-multiplexed.
With a switching board with conventional cell switches, however, the improvement in delay-throughput is limited and concatenation-multiplexed signals cannot be transferred properly.
Theoretically, the cell transfer with no conflict is possible as mentioned above. Practically, however, an optimum transfer path should be assigned for each cell path and for each time required to transfer a cell. Such an assignment is extremely difficult to do with software or hardware.
According to one of known devices to solve the above inconvenience, each S switch is equipped with a buffer memory for temporarily retaining some of the cells which have been inputted to each F switch simultaneously and destined to an identical T switch. The above-retained cells are sometimes delayed in transfer, in which case, the order of the cells is reversed after the cells are concatenation-multiplexed. As shown in FIG. 2b, cells C1 through C4 are inputted simultaneously and cells C5 through C8 are inputted simultaneously. However, the cells C2 and C6, which are retained in the buffer memory, are delayed in transfer as shown in FIG. 2c. Consequently, the original signal is not restored accurately.
According to another such device, the cells which may conflict are detected in advance, and which transfer paths will avoid cell conflict is determined. The cells are made to wait in the F switches until the determination is completed. This results in a drastic decrease in the transfer speed.
It is also known that more than 32 ATM switches 2302 are provided in order to send a smaller number of cells are each ATM switch cells and thus lower the possibility of cell conflict. This device requires a huge hardware system but cannot prevent the reversal of the cell order completely.